The exhaust gases of internal combustion engines may contain a number of combustion by-products, including NOx, particulate matter such as carbonaceous soot, and unburned hydrocarbon fuel. In particular, the exhaust gases of diesel engines contain significant amounts of NOx and soot, and NOx content is also high in the exhaust gases of so-called “lean burn” engines in which the fuel is combusted with excess oxygen in order to increase engine efficiency. The exhaust gas streams of lean burn engines may also contain significant amounts of oxygen.
The harmful effects of releasing NOx, soot and volatile organic compounds into the atmosphere are well documented. NOx participates in the generation of photochemical smog, acid rain and ozone. Soot particulates in the air contribute to poor visibility and respiratory disease.
In both compression (diesel) and spark ignition engines, EGR systems have been used to decrease NOx emissions. EGR systems can reduce NOx emissions of diesel engines by 40 to 50%, and greater reductions are possible by cooling the recirculated exhaust gas using an EGR cooling device such as a compact shell-and-tube heat exchanger. There are, however, limits on the amount of exhaust gas which can be reintroduced into the engine before power output and fuel economy are adversely impacted, and it has been found that the reintroduction of exhaust gases into the engine can significantly increase the soot content of the exhaust gas.
The presence of large amounts of soot in the exhaust gas can lead to fouling of the internal surfaces of the EGR cooler. It is believed that fouling decreases the performance of the EGR cooler by creating a low conductivity thermal barrier on the heat exchange surfaces, by causing surface roughness on the heat exchange surfaces which increases the pressure drop, and by constricting the passages through which the exhaust gases flow, especially in small diameter tubes.
It is known to use particulate traps and/or filters to reduce the soot content of an exhaust gas stream. Some of these traps and filters may include a plasma reactor which generates NO2 to react with the soot and convert it to gaseous CO and/or CO2. It is also known that the fouling of EGR coolers can be reduced or prevented by providing a particulate trap or filter upstream of an EGR cooler. However, these systems suffer from the disadvantage that an additional component is required in the exhaust gas stream for the purpose of reducing soot content. This adds to the cost and complexity of the exhaust system.
There remains a need for exhaust gas systems in which the fouling of the EGR cooler is partially or completely prevented in a manner which is simpler and more cost effective than in known systems.